(1) Field of the Invention
The invention relates to processes for packaging MEMS devices, and MEMS packages produced using the method, and more particularly, to a method of packaging MEMS devices using a flexible, foldable substrate.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS) devices are known which convert physical phenomena, such as pressure, acceleration, sound, or light, into electrical signals. Each type of MEMS device interacts with the world in a different way, and demands custom or at least semi-custom packaging solutions. So-called system-in-package techniques attempt to form an entire microsystem—which could include a microprocessor, communications components, actuators and sensors—within a single package. However, packaging of a MEMS device is totally different from packaging an integrated circuit. MEMS devices are categorically different from ICs despite sharing some fundamental processing technologies. Packaging is the biggest challenge for commercializing most MEMS devices. The term “MEMS package” is used in this document to imply a package including at least one MEMS device.
A MEMS device might function perfectly well in the controlled environment in which it was created. However, the device can be a real viable product only after it has been packaged with proven performance in a package. For example, the packaging stress can distort the sensitivity and the performance of the MEMS devices. MEMS devices include delicate movable structures which are easily damaged through fabrication and assembly processes. As such, the assembly yield of a MEMS package is often a challenging target to meet.
The packaging requirements of MEMS devices are complex because the devices need to interact with the physical phenomenon and yet the devices need to be protected from the environment. As such, exotic package structures with specialized assembly techniques and unique packaging materials are employed for MEMS devices. Packaging is usually responsible for at least 60 percent of the cost of a MEMS device, and sometimes as much as 85 percent. Thus, it has been recognized that a low cost packaging solution with robust assembly is needed to promote the use of MEMS devices.
A variety of packages are known for packaging various MEMS products. “The introduction of MEMS packaging technology”, by C. T. Hsieh et al, Proceedings of the 4th International Symposium on Electronic Materials and Packaging, 4–6 Dec. 2002, pp. 300–306, describes (for example with reference to its FIG. 3) various metal packages, such as TO8 (or round) headers, and butterfly and platform packages. Metal packages provide good heat dissipation capability and shielding of electrical radiation. The TO8 header is commonly fabricated from Kovar alloy to reduce the thermo-mismatch between the packaging material and the silicon etched devices.
“A new approach for opto-electronic/MEMS packaging” by R. Keusseyan et al, Proceedings of the 52nd Electronic Components and Technology Conference, 28–31 May 2002, pp. 259–262, describes (for example with reference to its FIG. 3) ceramic or LTCC packages. These are low cost, high reliability, gastight and multi-layer packaging architectures. An example of a ceramic package is an IR bolometer which is produced at high processing temperature in a vacuum seal environment.
“Challenges in the packaging of MEMS”, by C. B. O'Neal, et al, Proceedings of the International Symposium on Advanced Packaging Materials: Processes, Properties and Interfaces, 14–17 Mar. 1999, pp. 41–47, compares (for example with reference to its FIG. 1) IC packages and MEMS pressure sensor packages. Plastic/lead frame packages are commonly employed in IC packaging and commonly classified as pre-molded and post-molded packages. The difference between pre-molded and post-molded packages is that pre-molded packages comprise a package body including a hollow cavity into which the IC is placed and then covered with a sealing cap, whereas in post-molded packages, the package body is molded over the assembly after the IC has been attached. Both these alternatives apply also to MEMS devices. For example, a post-molded package can be used for ICs or wafer capped accelerometers, while a pre-molded package can be used for a pressure sensor or microphone packaging. The plastic package offers a low cost packaging option. However, the required mold tooling is often expensive and time consuming, making it inflexible to meet fast changing needs from end-users' applications. Others key issues are that the plastic has very poor matching of thermal expansion with silicon and is also susceptible to moisture ingression.
Wafer level packaging (WLP) is a niche method for MEMS packaging. It involves an extra fabrication process where a micromachined wafer is bonded to a second wafer which has appropriate cavities etched into it. Once bonded, the second wafer creates a protective silicon cap over the micro-machine structure. This method leaves the microstructure free to move within a vacuum or an inert gas atmosphere. The bond is hermetic and therefore prevents moisture contamination and hence failure of the microstructure. WLP is discussed in “Considerations for MEMS packaging”, Biye Wang, Proceedings of the Sixth IEEE CPMT Conference on High Density Microsystem Design and Packaging and Component Failure Analysis, 30 Jun.–3 Jul. 2004, pp. 160–163 and “Overview and development trends in the field of MEMS packaging”, H. Reichl et al, The 14th IEEE International Conference on Micro Electro Mechanical Systems, 21–25 Jan. 2001, pp. 1–5.
U.S. Pat. No. 6,781,281 to Minervini and U.S. patent application Ser. No. 2002/0102004 A1 to Minervini discloses a MEMS microphone package in which a MEMS transducer element, an IC die and other capacitor components are located on a first multi-layer FR4 printed circuit board (PCB). A second multi-layer FR4 PCB is used as a cover. The two FR4 boards are spaced apart by a third FR4 board, which is cut to include a window which is placed around the components on the first PCB. Thus, the three PCBs cooperate to house and shield the transducer element, the IC die and other capacitor components. Compared with plastic/lead frame packages, such a package enables a larger batch operation, requires minimal hard tooling and has better match of thermal expansion with end user's PCB. Nevertheless, the mixing of the transducer element, the IC die and other electronic components on the same FR4 PCB substrate still presents difficulties in operating a high yield assembly process. Furthermore, a multi-layer FR4 PCB is not a cheap packaging material.
PCT patent application PCT/SG2005/000034 filed on 8 Feb. 2005 provides a method and package in which at least one MEMS device is mounted on a first flex substrate, and one or more electronic components are mounted on a second substrate. The two substrates are then joined mechanically in parallel with a spacer element between them and connected electrically by electrical connecting elements. The substrates sandwich the spacer element, the electrical connecting elements, the MEMS device and the one or more electronic components between them. The advantage of this method is that the process of mounting the MEMS device can be dealt with and performed separately from the process for mounting other IC and electronics components, thus making the assembly easier and higher yield. However, interconnecting the two substrates is a challenging task.